This is a competing renewal application of a MERIT award currently in its 36th year of funding. The long-term objectives of these studies have been to elucidate the mechanisms by which systemic hormones and local growth factors regulate the normal development of the mammary gland, and to determine how these regulatory mechanisms have deviated in breast cancer. More than a decade ago our laboratory and others presented evidence that steroid hormone-induced proliferation of mammary ductal epithelial cells was regulated by paracrine mechanisms. Recent studies have indicated that steroid hormone regulation of mammary stem and progenitor cells in both the normal mammary gland and luminal breast cancers is also mediated by local paracrine mechanisms involving Wnts, fibroblast growth factors and Rank ligand. Our current studies combine gain- and loss-of function mouse genetic experiments using transgenic, conditional knockout, and lentivirally-transduced mammary epithelial cells and transplantation into the cleared mammary fat pad with the in situ analysis of signal transduction pathways. They will continue to focus on the understanding of both paracrine, juxtacrine and autocrine mechanisms of cell fate determination in the murine mammary gland with emphasis on the mechanisms regulating stem/progenitor cell self-renewal, normal ductal morphogenesis, and early progression in breast cancer. To accomplish these goals we propose: 1. To elucidate the role of the canonical and non-canonical Wnt pathways in mammary stem/progenitor cell self-renewal and survival. 2. To elucidate the molecular mechanisms of FGFR 1 and FGFR2 action on mammary stem and progenitor cells. 3. To understand the mechanisms of Wnt and Fgf pathway cooperativity in mammary stem/progenitor self- renewal and how these are altered in breast cancer. 4. To delineate the role of specific C/EBPss protein isoforms in both stem cell self-renewal and luminal/alveolar cell fate determination 5. To elucidate the role of noncoding RNAs(ncRNAs) in mammary gland development and breast cancer. Our understanding of the biology and developmental genetics of the normal mammary gland is a barrier to progress...a more complete understanding of the normal mammary gland at each stage of development....will be a critical underpinning of continued advances in detecting, preventing and treating breast cancer